1. Field of the Invention
This invention relates to bearings and, in particular, to cylindrical squeeze bearings where one vibrating bearing member is a bimorph element.
2. Description of the Prior Art
The general principles of operation of squeeze bearings are set forth in an article entitled "Compressible Squeeze Films and Squeeze Bearings" by E. O. Salbu in the June 1964 issue of the Trans. ASME Journal of Basic Engineering. Since that time, the art has suggested various configurations for bearing members along with improvements in the bearing materials and in construction of the bearing elements per se. Interest has developed in the use of tubular shaped squeeze bearing systems for magnetic disk files, particularly in the carriage mechanism which functions to guide the magnetic transducer of the disk file as it is moved between magnetic tracks on a rotating magnetic disk. See, for example, commonly assigned copending application Ser. No. 82/01828, filed Dec. 30, 1982 and also commonly assigned copending application Ser. No. 273,280, filed June 12, 1981.
The prior art has shown that considerable improvement in the bearing quality, i.e., the load carrying capacity and the bearing stiffness, is obtained when one of the tubular or cylindrical bearing elements is what is referred to as a bimorph element. Generally, a bimorph element comprises a cylinder of piezoceramic material, such as PZT-4, which is integral with a cylinder of bearing material such as Pyrex glass. The bimorph element functions as the energizing transducer for the squeeze bearing by causing the bearing surface to vibrate or oscillate relative to an adjacent bearing surface of the other bearing member element to produce the air pumping action necessary to establish the squeeze bearing system.
It is known that the piezoceramic material in the energizing transducer of the cylindrical bearing element is subject to mechanical failure due to fatigue of the material which is subjected to cyclic loading during normal operation. The high frequency stress reversals (tensile to compressive and back to tensile, etc.) are especially undesirable since they establishes a fairly low nominal allowable stress. The number of stress reversal cycles, which may reach 100 million cycles in less than an hour at ultrasonic frequencies, makes it necessary to use an endurance stress limit in the design calculation of squeeze bearing members. The maximum stresses encountered depend upon deflection of the squeeze bearing at the resonance frequency and the associated electric field strength (i.e., the applied voltage for a given geometry). Since mechanical failure is, in fact, the common failure mode, the endurance limit establishes an upper bound for deflections and applied voltage for the squeeze bearing, thus limiting obtainable load capacity and bearing stiffness.
It has been found that by limiting stress reversals and operating the piezoelectric material in compression at all times, the incidence of fatigue failure can be greatly reduced. By prestressing the piezoceramic material radially and circumferentially so that it is in compression, the cyclic tensile stresses, which are also radial and circumferential, are minimized, if not automatically eliminated.
The present invention is directed to a cylindrical squeeze bearing system employing a bimorph transducer in which concern is greatly reduced for mechanical failure of the bearing as a result of fatigue of the piezoceramic material.